drm/i915: Warn if irq_mask isn't ~0 during vlv/cvh display irq postinstall

We expect vlv_display_irq_reset() to have been called prior to
vlv_display_irq_postinstall() so let's WARN if that isn't the case.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460382992-28728-8-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -3321,6 +3321,9 @@ static void vlv_display_irq_postinstall(struct drm_i915_private *dev_priv)
 		I915_DISPLAY_PIPE_B_EVENT_INTERRUPT;
 	if (IS_CHERRYVIEW(dev_priv))
 		enable_mask |= I915_DISPLAY_PIPE_C_EVENT_INTERRUPT;
+
+	WARN_ON(dev_priv->irq_mask != ~0);
+
 	dev_priv->irq_mask = ~enable_mask;
 
 	GEN5_IRQ_INIT(VLV_, dev_priv->irq_mask, enable_mask);